In 20 healthy volunteers double-blind randomized study was done to examine the effect of co-dergocrine (Hydergine) on platelet function. All the volunteers were pretreated with 3 X 1 placebo for 1 week, followed by the randomized study in which 10 volunteers each received drug or placebo for 6 weeks. Various platelet function parameters, such as the peripheral platelet count, plasma thromboxane B2, platelet sensitivity to the antiaggregatory prostaglandins PGI2, PGE1 and PGD2, ADP-induced aggregation, collagen-induced aggregation, the WU-test, the platelet proteins platelet factor 4 and beta-thromboglobulin, malondialdehyde, circulating endothelial cells and the intracellular c-AMP level in platelets were examined. Treatment with co-dergocrine decreased platelet activity to a significant extent, as shown by a number of platelet function parameters, such as thromboxane B2, WU-test, beta-thromboglobulin, platelet factor 4, malondialdehyde and ADP-induced aggregation. The findings suggest that co-dergocrine might be able to decrease platelet activity and improve interaction with the walls of to a blood-vessel significant degree.